HP ProCurve 9304M Installation And Configuration Manual page 229

NOTE: This rapid convergence will not occur on ports connected to shared media devices, such as hubs. To take
advantage of the rapid convergence provided by 802.1W, make sure to explicitly configure all point-to-point links in
a topology.
The convergence provided by the standard 802.1W protocol occurs more rapidly than the convergence provided
by previous spanning tree protocols because:
• Classic or legacy 802.1D STP protocol requires a newly selected Root port to go through listening and
learning stages before traffic convergence can be achieved. The 802.1D traffic convergence time is
calculated using the following formula:
2 x FORWARD_DELAY + BRIDGE_MAX_AGE.
If default values are used in the parameter configuration, convergence can take up to 50 seconds. (In this
document STP will be referred to as 802.1D.)
• RSTP Draft 3 works only on bridges that have Alternate ports, which are the precalculated "next best root
port". (Alternate ports provide back up paths to the root bridge.) Although convergence occurs from 0 – 500
milliseconds in RSTP Draft 3, the spanning tree topology reverts to the 802.1D convergence if an Alternate
port is not found.
• Convergence in 802.1w bridge is not based on any timer values. Rather, it is based on the explicit
handshakes between Designated ports and their connected Root ports to achieve convergence in less than
500 milliseconds.
Bridges and Bridge Port Roles
A bridge in an 802.1W rapid spanning tree topology is assigned as the root bridge if it has the highest priority
(lowest bridge identifier) in the topology. Other bridges are referred to as non-root bridges.
Unique roles are assigned to ports on the root and non-root bridges. Role assignments are based on the following
information contained in the Rapid Spanning Tree Bridge Packet Data Unit (RST BPDU):
• Root bridge ID
• Path cost value
• Transmitting bridge ID
• Designated port ID
802.1W algorithm uses this information to determine if the RST BPDU received by a port is superior to the RST
BPDU that the port transmits. The two values are compared in the order as given above, starting with the Root
bridge ID. The RST BPDU with a lower value is considered superior. The superiority and inferiority of the RST
BPDU is used to assign a role to a port.
If the value of the received RST BPDU is the same as that of the transmitted RST BPDU, then the port ID in the
RST BPDUs are compared. The RST BPDU with the lower port ID is superior. Port roles are then calculated
appropriately.
The port's role is included in the BPDU that it transmits. The BPDU transmitted by an 802.1W port is referred to as
an RST BPDU, while it is operating in 802.1W mode.
Ports can have one of the following roles:
• Root – Provides the lowest cost path to the root bridge from a specific bridge
• Designated – Provides the lowest cost path to the root bridge from a LAN to which it is connected
• Alternate – Provides an alternate path to the root bridge when the root port goes down
• Backup – Provides a backup to the LAN when the Designated port goes down
• Disabled – Has no role in the topology
Configuring Spanning Tree Protocol (STP) and Advanced STP Features
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